1. Field of the Invention
The present invention relates to a cathode-ray tube apparatus.
2. Description of the Related Art
Recently, there is an increasing demand for flattening and enlarging the display screen of a cathode-ray tube apparatus. In order to satisfy this demand while ensuring predetermined mechanical strength, it is necessary to increase the thickness of a glass bulb for a cathode-ray tube, which consequently leads to an increase in weight.
JP2002-237266A describes a glass funnel for a cathode-ray tube, capable of simultaneously realizing sufficient strength withstanding vacuum breakdown and reduction in weight. In this funnel, a difference in level is formed on an outer surface of the funnel in the vicinity of a portion connected to a front panel in such a manner that the thickness is large at the portion connected to the front panel, and is small in a region on a neck portion side from the connected portion.
Generally, in a cathode-ray tube apparatus, an electron beam is deflected so as to scan (overscan) a region larger than a screen display region. When an electron beam strikes an inner wall surface of the funnel during overscanning, the electron beam reflected from the inner wall surface is incident upon a phosphor screen to allow a phosphor to emit light, whereby so-called cone halation occurs. This degrades image quality. The electron beam is likely to strike the inner wall surface of the funnel in the vicinity of a region of the funnel opposed to a deflection yoke.
In order to prevent the cone halation, the internal size of the funnel may be enlarged so that the electron beam does not strike the inner wall surface of the funnel even during overscanning.
However, in order to enlarge the internal size of the funnel in the above-mentioned conventional funnel in which the thickness is small in a region on the neck portion side from the difference in level, it is necessary to further reduce the thickness of the funnel. In the funnel, in order to limit an X-ray leakage amount to a predetermined value or less, it is necessary to use glass containing lead and maintain a predetermined thickness. Thus, when the thickness of the funnel is reduced, there is a new problem that an X-ray leakage amount increases.
In order to enlarge the internal size of the funnel while keeping a predetermined thickness, it is necessary to increase the external size of the funnel. However, there is an upper limit of the external size of the funnel in order to avoid the interference with the deflection yoke to be mounted on an outer circumferential surface of the funnel. If the internal size of the deflection yoke is increased so as to increase the external size of the funnel, the distance between the deflection yoke and the electron beam is enlarged to cause an increase in the required deflection power.